The invention relates to channel selection in a mobile radio communication system, and more particularly to methods and apparatuses for selecting a channel to be used in a mobile radio communication system cell, the selection being made in a way that reduces the occurrence of co-channel interference.
In a cellular mobile radio communication system, the geographic area served by the system is divided-into geographically defined cells, each of which is serviced by a base station. In the system, there is a finite number of carrier frequencies that are available for use during communications. A frequency group, consisting of a subset of all of the available frequencies, is assigned to each cell for that cell's use during communications. However, because the number of frequency groups is limited, it is necessary to reuse them within the area served by the system. Because the use of a particular frequency by two different cells can result in co-channel interference, possibly reducing the carrier-to-interference ratio below an acceptable quality threshold, an attempt is made to assign frequency groups to cells in a manner that results in any given cell using a different frequency group from its neighbors. A group of cells is called a cluster when the combination of their individually assigned frequency groups includes all available frequencies assigned to the system. There are usually a number of clusters within a system, each repeating a particular frequency allocation, called a reuse pattern.
Cells to which the same frequency group has been allocated are called co-channel cells. It follows from this description that each co-channel cell is assigned to a different cluster from all other co-channel cells.
It is desirable to try to limit the effect of co-channel interference which arises from the simultaneous use of a particular frequency by two different co-channel cells. This can be achieved by assigning frequencies so that the distance between co-channel cells is maximized. However, maximizing this distance means assigning fewer frequencies to each cell. This may conflict with system requirements to increase the capacity of each cell by increasing the number of frequencies assigned thereto.
In a normal reuse pattern, such as the 21-cell frequency repeat pattern, the average expected co-channel interference in a given cell is based on the assumption that all co-channel frequencies in all co-channel cells are used. That is, for a cell having a frequency group f.sub.A containing frequencies f.sub.1, f.sub.22, f.sub.43, f.sub.64, . . . , the average expected co-channel interference in a given cell is calculated by assuming that each of the cell's frequencies are simultaneously in use by the cell and by the co-channel cells (i.e., those cells to which the same frequency group has been assigned), even though this is often not the case.
For example, reference is made to FIG. 1, which shows a system having a 7-cell frequency repeat pattern. Two cell clusters, each consisting of seven cells, are shown. The first cell cluster 100, consists of cells labeled C.sub.11, C.sub.12, C.sub.13, C.sub.14, C.sub.15, C.sub.16, and C.sub.17. The second cell cluster 102 consists of cells labeled C.sub.21, C.sub.22, C.sub.23, C.sub.24, C.sub.25, C.sub.26, and C.sub.27. The showing of only two cell clusters is done here solely for convenience. It should be understood that, in practice, a mobile telephone system may have more than two cell clusters. It should also be understood that the following discussion is not limited to systems having a 7-cell frequency repeat pattern, but is equally applicable to all repeat patterns, including the popular 12-cell and 21-cell frequency repeat patterns.
As shown in FIG. 1, each cell is assigned a frequency group, denoted here as the letter "f" with a letter subscript. For example, each of cells C.sub.11 and C.sub.21 has been assigned the frequency group f.sub.A.
Specific frequencies are denoted here as the letter f with a numeric subscript. In the 7-cell frequency repeat pattern shown in FIG. 1, frequency group f.sub.A consists of the frequencies f.sub.1, f.sub.8, f.sub.15, f.sub.22 . . . , frequency group f.sub.B consists of the frequencies f.sub.2, f.sub.9, f.sub.16, f.sub.23 . . . , and so on.
A mobile station, M, in cell C.sub.11 operates on one of the frequencies in group f.sub.A, such as f.sub.22. The call may or may not experience co-channel interference from another user located in cell C.sub.21, depending on whether the same frequency, in this case f.sub.22, is being utilized in cell C.sub.21.
In high traffic, typically 70-80% of the traffic frequencies, or channels, are in use. If traffic utilization rises higher than 80%, the user's perceived congestion becomes intolerable. It follows that, in high traffic, the probability that a particular channel is simultaneously being used in both cells C.sub.11 and C.sub.21 is high. This simultaneous use results in a high expected co-channel interference.
Referring now to FIG. 2, the middle cell C.sub.11, first shown in FIG. 1, is illustrated along with its six closest co-channel cells, C.sub.21, C.sub.31, C.sub.41, C.sub.51, C.sub.61, and C.sub.71. It should be understood that each of the illustrated co-channel cells is located at the center of a seven-cell cluster, similar to the first cell cluster 100. However, for simplicity, the remaining cells from each cluster, which are located between those shown, are not illustrated.
As previously stated, each co-channel cell has been assigned the same frequency group. For the sake of convenience in the remainder of this discussion, references made to particular frequencies within a frequency group will assume that the frequencies within the group are numbered sequentially, from 1 to n, where n is an integer. It will also be understood that each particular frequency receives the same numbering assignment in each co-channel cell.
FIG. 3 illustrates a method for assigning an available frequency for use within a cell. For the purpose of example, a frequency group 300 is shown which has twenty-one sequentially numbered frequencies. As shown by the arrow 302, frequencies are assigned by taking the leftmost frequency that is available in the list. That is, priority assignments have been made to each of the frequencies, such that priority of selection increases the closer to the left a given frequency is in the list. Consequently, frequency 1 is first investigated to see whether it is available/suitable. If it is not, the search continues to frequency 2, and so on, until an available/suitable frequency is found. If no frequency is found, congestion arises.
In the method described above with reference to FIG. 3, if all cells have the same list and the same algorithm for selecting frequencies, then frequency 1 will often be used and frequency 21 will almost never be used. Consequently, the co-channel interference conditions on frequency 1 will generally be as poor or good as the system is designed for, and frequency 21 will often provide good co-channel interference values in the rare instances in which it is assigned for use by a cell. Thus, even when each cell experiences low traffic, the co-channel interference experienced by the few assigned frequencies will still be high, since the same frequencies are likely to be assigned by each cell.
The reader will recognize that the deficiencies in the method just described are present when each frequency is assigned to only one channel in a system called Frequency Division Multiple Access (FDMA), and also when each frequency is assigned to more than one channel, in a Time Division Multiple Access (TDMA) system.
Therefore, it is an object of the present invention to select frequencies for use in co-channel cells in an FDMA mobile communication system in such a manner that the quality of a connection (as measured by the amount of co-channel interference on a frequency) is improved when the traffic level is less than a maximum capacity.
Another object of the invention is to select frequencies for use in co-channel cells in such a manner that the cell hardware associated with a particular frequency is not unevenly used, relative to the cell hardware associated with other frequencies in the cell.
A further object of the invention is to select frequencies for use in co-channel cells operating in a synchronized TDMA environment in such a manner that the quality of a connection is improved when the traffic level is less than a maximum capacity.
A still further object of the invention is to select channels for use in co-channel cells operating in a synchronized TDMA environment in such a manner that the quality of a connection is improved when the traffic level is less than a maximum capacity.
Yet another object of the invention is to select channels for use in co-channel cells operating in a TDMA environment in an asynchronous state wherein the TDMA structures of co-channel cells are sliding slowly relative to each other and the time differences are known. This state is referred to hereafter as a pseudo-synchronized TDMA environment or simply pseudo-synchronization.